It is desirable that air taken into a combustion chamber of an internal combustion engine has an adequate swirl so that it is well mixed with fuel. This swirl is generally obtaihed by using a spirally shaped intake port to give a directivity of air flow. When the engine is lightly loaded, a swirl of air occurs to facilitate mixing with fuel thereby improving the fuel efficiency. However, when the engine is heavily loaded, intake air flow is impeded by the spirally shaped intake port depending on its shape which will result in decreased filling efficiency. It is therefore necessary to provide effective means for preventing such a problem. For this purpose, the intake port has been provided with a parted port by a partition plate or a separate port having a valve to vary the magnitude of swirl.
However, such provision of the parted port or the separate port with valve makes the the intake port complex in structure which is disadvantageous in view of operation, manufacture and cost.